Improve support for alternative Log4j 2 configuration file formats

This commit adds support for configuring Log4j 2 with YAML. It also
improves the existing support for configuring Log4j 2 with JSON.

Previously, Log4J2LoggingSystem returned a hard-coded list of standard
config locations that includes both JSON and XML file suffixes. Log4j 
2’s support for JSON configuration files requires Jackson’s ObjectMapper
to be on the classpath so, in its absence, the standard config locations
were incorrect.

This commit updates Log4J2LoggingSystem to return an array of standard
config locations based on what’s on the classpath. It also updates the
documentation to describe the additional dependencies that are required
to enable YAML or JSON-based configuration.

Closes gh-2239

spring-boot-dependencies/pom.xml

@@ -445,6 +445,11 @@
 				<artifactId>jackson-dataformat-xml</artifactId>
 				<version>${jackson.version}</version>
 			</dependency>
+			<dependency>
+				<groupId>com.fasterxml.jackson.dataformat</groupId>
+				<artifactId>jackson-dataformat-yaml</artifactId>
+				<version>${jackson.version}</version>
+			</dependency>
 			<dependency>
 				<groupId>com.fasterxml.jackson.datatype</groupId>
 				<artifactId>jackson-datatype-joda</artifactId>